CD8 (cluster of differentiation 8) is a transmembrane glycoprotein which is a specific marker for a subclass of T-cells (which includes cytotoxic T-cells). CD8 assembles as either a heterodimer of the CD8 alpha and CD8 beta subunits or a CD8 alpha homodimer. The assembled dimeric CD8 complex acts as a co-receptor together with the T-cell receptor (TCR) to recognize antigen presentation by MHC class I cells. CD8 plays a role in the development of T-cells and activation of mature T-cells. Changes in T-cell localization can reflect the progression of an immune response and can occur over time.